If you purchased the motherboard with a credit card, ask the credit card company if your card has any type of purchase protection.

You could also try to explain to the credit card company exactly what happen and tell them the store refused to RMA the item, they might be willing to "charge back" the purchase to the store, The store will then contact you in a hurry trying to resolve this issue.

If you paid cash / debit you might be SOL. I try to use quality credit card for all my major purchases, it gives you a lot more protection (specially online purchases).

"Hello, I've installed my graphic card in a bad and wrong way, breaking things trying. I told the computer store's staff that it was this way when I bought it and they didn't believe me. I voided my warranty and they refuse to exchange or refund me, this is unacceptable. I will now stop buying ASUS products and also stop shopping at this horrible computer store because of my own negligence."